The present invention relates to a semiconductor device having a pressure contact structure and, more particularly, to an improvement of a semiconductor device, such as a transistor, gate turn-off thyristor or high speed thyristor, having a structure wherein main electrodes and control electrodes of a semiconductor device are alternately formed.
In general, the wiring or electrode layer of a semiconductor device is comprised of aluminum of high conductivity. However, since aluminum is soft and has a low melting point, it cannot always be used as the most suitable electrode material in the pressure contact structure of a high powered semiconductor device, as will be explained later. Therefore, the material and process employed in manufacturing such a pressure contact structure of a high powered semiconductor device must be properly selected, from a different point of reference than in the case of a low powered semiconductor device.
FIG. 1 is a sectional view showing the structure of a planar type power transistor, as one example of a power semiconductor device.
Referring to FIG. 1, reference numeral 1 denotes a ceramic case; 2, an npn transistor; 3 and 4, heat buffer plates made of Mo or W, respectively; and 5 and 6, Cu external electrodes, respectively. The external electrode 5 is brought into pressure contact with each of a plurality of emitter electrodes 7, through the heat buffer plate 4. The external electrode 6 is connected to a substrate 8 of the transistor 2, through the heat buffer plate 3. The substrate 8 is bonded to the heat buffer plate 3 through a bonding metal layer 11 such as a solder. One of a plurality of base electrodes 9 of the transistor 2 is connected to a lead 10. The lead 10 extends outside the ceramic case 1.
In the power transistor having the structure described above, the emitter electrodes 7 are made of an Al film having a thickness of 10 .mu.m. These emitter electrodes 7 are formed on a plurality of emitter regions which protrude upward from the transistor 2 having a diameter of about 40 mm. The emitter electrodes 7 are brought into pressure contact with the heat buffer plate 4 at a pressure level of about 1.0 to 1.5 tons. The emitter electrodes 7 have a level difference of about 20 .mu.m with respect to the base electrodes 9. The emitter electrodes 7 and the base electrodes 9 are alternately formed in such a way that the emitter electrodes 7 are spaced from about 200 to 300 .mu.m apart from each other in the direction of the width.
However, in the power semiconductor device described above, although the Al emitter electrodes 7 and the external electrode 5 have the heat buffer plate 4 sandwiched therebetween, the emitter electrodes 7 are affected by heat fatigue, due to a heat cycle of intermittent operation of the transistor. As a result, any or several of the emitter electrodes 7 may be inclined in the transverse direction and, hence, may extend through a passivation film (not shown) formed on the base electrodes 9 and reach the corresponding base electrode 9. Such an emitter electrode 7 then fusion bonds to the corresponding base electrode 9. As a result, the base and the emitter regions are short-circuited.